Light-emitting apparatus, phosphor, and method of producing it

ABSTRACT

A light-emitting apparatus composed of a light source that emits primary light and a phosphor that absorbs the primary light and emits secondary light offers high brightness, low power consumption, and a long lifetime while minimizing adverse effects on the environment. The phosphor is formed of a III-V group semiconductor in the form of fine-particle crystals each having a volume of 2 800 nm 3  or less. The light emitted from the fine-particle crystals depends on their volume, and therefore giving the fine-particle crystals a predetermined volume distribution makes it possible to adjust the wavelength range of the secondary light.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a light-emitting apparatus, andmore particularly to a light-emitting apparatus that irradiates aphosphor with primary light emitted from a primary light source and thatthereby produces secondary light having longer wavelengths than theprimary light.

[0003] 2. Description of the Prior Art

[0004] GaN-based semiconductors are direct-transition semiconductorswith band gaps ranging from 0.9 eV or 1.8 eV to 6.2 eV, and thus theymake it possible to realize light-emitting devices that emit light in awide band ranging from visible to ultraviolet regions. For this reason,in recent years, GaN-based semiconductors have been receiving muchattention and have been actively developed. The reason that the lowerlimit of the band gap is given as 0.9 eV or 1.8 eV above is that theband gap of InN has not yet been definitely determined between twotheories that support 0.9 eV and 1.8 eV respectively.

[0005] As is widely practiced, such a GaN-based light-emitting device isused as an excitation light source to produce white light by mixingtogether phosphorescence of different colors emitted from red, green,and blue phosphors irradiated with light emitted from the GaN-basedlight-emitting device.

[0006] It has also been proposed to use a GaN-based light-emittingdevice in a full-color image display apparatus (Japanese PatentApplication Laid-Open No. H8-63119). In this full-color image displayapparatus, either phosphors each emitting phosphorescence of one ofthree primary colors, namely red, green, and blue, are excited by aGaN-based light-emitting diode array arranged on a substrate, orphosphors each emitting red or green phosphorescence are excited by aGaN-based light-emitting diode array and blue light is emitted directlyfrom GaN-based light-emitting diodes.

[0007] On the other hand, next-generation light-emitting apparatuses aredesired to offer high brightness combined with low power consumption.The brightness and power consumption of a light-emitting apparatusdepend on the output power and quantum efficiency of its excitationlight source and on the quantum efficiency of its phosphors. Thus, thephosphors are desired to have as high quantum efficiency as possible.Moreover, the resolution of a full-color display apparatus depends onthe size of its pixels, and therefore, in a case where a phosphorescentsurface is formed by applying a phosphorescent material on a surface, itis necessary to reduce the size of the crystal particles of thephosphorescent material to suit the size of the pixels.

[0008] However, conventionally available phosphors have quantumefficiency of 10% or lower, and therefore, to obtain higher brightness,it is necessary to increase the light output power of the excitationlight source. Inconveniently, this increases power consumption andshortens the lifetime of the excitation light source. For this reason,it has to date been difficult to realize a light-emitting apparatus thatuses a GaN-based light-emitting device as an excitation light source andthat offers high brightness combined with low power consumption and along lifetime.

[0009] Recently, it has been observed that reducing the size of acrystal down to the exciton Bohr radius (hereinafter, such a crystalwill be referred to as a “nano-crystal”) causes, owing to the quantumsize effect, trapping of excitons and an increase in the band gap (J.Chem. Phys., Vol. 80, No. 9, p. 1984). It has been reported that somesemiconductors of such size exhibit photoluminescence with high quantumefficiency (Phys. Rev. Lett., Vol. 72, No. 3, p. 416, 1994; MRS bulletinVol. 23, No. 2, p.18, 1998; and U.S. Pat. No. 5,455,489).

[0010] Now, this effect will be described in the case of Mn-doped ZnS(ZnS:Mn), which is taken up as an example here for easy comparisonbecause its light emission wavelength does not vary under the quantumsize effect. Table 1 shows the results of comparison of the brightnessof light emission obtained when ZnS:Mn nano-crystals having theirsurface treated with methacrylic acid and bulk ZnS:Mn particles having aparticle size of 1 μm or greater were excited with the same ultravioletlamp. Table 1 shows that the ZnS:Mn nano-crystals offer brightness closeto five times the brightness offered by the bulk ZnS:Mn particles. TABLE1 Nano-crystals Bulk Particles Brightness 69 cd/m² 14.2 cd/m²

[0011] How this high quantum efficiency physically relates to thequantum size effect has not yet been definitely explained, but thepossible factors that are considered to be involved include an increasein the intensity of excitons which results from the formation ofelectron-hole pairs, a decrease in the state density that does notcontribute to light emission which results from the quantization ofenergy levels, a variation in the crystal field near the center of lightemission which results from distortion of the crystal lattice, andsurface treatment of the crystals. Which of these factors contributeseffectively to light emission efficiency is not clear, but lightemission efficiency has been reported to increase in crystals havingsizes smaller than the exciton Bohr radius, which will be describedbelow.

[0012] Here, the exciton Bohr radius indicates the extent of theprobability of existence of excitons, and is given by 4π ε₀ h²/me²(where “ε₀” represents the low-frequency dielectric constant of thematerial, “h” represents the Planck constant, “m” represents the reducedmass obtained from the effective masses of an electron and a hole, and“e” represents the electric charge of an electron). For example, theexciton Bohr radius of ZnS is about 2 nm, and that of GaN is about 3 nm.

[0013] A most typical example of the quantum size effect is an increasein the band gap. FIG. 1 shows the dependence of the band gap on thecrystal size as calculated on the basis of the theory by L. E. Brus etal. The intrinsic band gap of ZnS is about 3.5 eV, and therefore thequantum size effect is expected to increase in the range where thediameter is smaller than about 8 nm. This diameter is that of a crystalhaving a radius equal to twice the exciton Bohr radius.

[0014] Accordingly, by using a phosphor formed of crystals having a sizeequal to or smaller than twice the exciton Bohr radius, it is possibleto exploit the contribution of the quantum size effect to lightemission. That is, by varying the size of nano-crystals, it is possibleto obtain different phosphorescence wavelengths. As nano-crystalmaterials having high quantum efficiency other than ZnS, there areactively being studied II-VI group materials such as CdSe.

[0015] Moreover, as shown in FIG. 2, a CdSe nano-crystal capped with ZnShas a quantum well structure, in which electron-hole pairs are stronglytrapped inside the nano-crystal and thus undergo recombination. Thismaterial offers light emission efficiency higher by an order or more ofmagnitude than that of an uncapped CdSe nano-crystal, and offers quantumefficiency of about 50%.

[0016] A display apparatus and an illumination apparatus using a II-VIgroup nano-crystal material has been proposed (Japanese PatentApplication Laid-Open No. H11-340516).

[0017] However, II-VI group materials have the following disadvantages.The result shown in Table 1 is that obtained when the nano-crystals weresubjected to surface treatment using methacrylic acid. However, incrystals that are not subjected to surface treatment, excited electronsare captured by the dangling bond of ions existing on the surface andundergo non-radiation recombination. This greatly reduces light emissionintensity. For example, as shown in Table 2, in ZnS:Mn nano-crystals ofwhich the surface is not treated with methacrylic acid, the danglingbond on the surface of the crystals is not effectively terminated, withthe result that the light emission intensity of these nano-crystals isfar lower than that of the sample of which the surface is treated. Thus,II-VI group nano-crystals require a special process to stabilize itssurface. TABLE 2 Surface Treatment Methacrylic Acid No Brightness 69cd/m² 9.4 cd/m²

[0018] Moreover, II-VI group materials contain toxic substances such asCd and Se, and therefore using II-VI group materials in light-emittingapparatuses and image display apparatuses is a matter of great concernfrom the environmental perspective.

SUMMARY OF THE INVENTION

[0019] An object of the present invention is to provide a light-emittingapparatus that uses a stable, environmentally friendly material and thatoffers high brightness combined with low power consumption and a longlifetime.

[0020] To achieve the above object, the inventors of the presentinvention studied the use of nano-crystals of a III-V group nitridecompound semiconductor, which is stable as a material and which hasminimum adverse effects on the environment.

[0021] A description will be given of a case where InN nano-crystals areused as a phosphor. The intrinsic band gap of InN at room temperature is0.9 eV or 1.8 eV. However, when the size of the crystals is reduced toabout 14 nm along each side, the band gap increases under the quantumsize effect. The dielectric constant of InN is unknown, and thereforethe Bohr radius of InN is unknown. On the other hand, GaN is known tohave a Bohr radius of 3 nm, and therefore InN is considered to have aBohr radius not greatly different therefrom. Thus, the quantum effect isconsidered to appear at a particle size about twice that Bohr radius

[0022] The surface of this phosphor is stable, and offers high quantumefficiency without special surface treatment. Moreover, the elementsthat the phosphor contains are mostly III group elements and nitrogen;that is, it contains no toxic elements, an advantage from theenvironmental perspective.

[0023] Moreover, the half width of the phosphorescence of InNnano-crystals is about 20 nm, which is extremely narrow as compared withthat of a common bulk phosphor, for example 60 nm of ZnS:Ag. This helpsrealize light-emitting apparatuses and image display apparatuses withhigh color quality.

[0024] Moreover, by reducing the crystal size of InN, it is possible torealize not only red but also green and blue phosphors with InNnano-crystals. Thus, it is possible to obtain phosphorescence coveringfrom red to blue with a single material, and thus to realize afull-color display or the like with a single phosphorescent material.

[0025] Furthermore, by laying a red phosphor, a green phosphor, and ablue phosphor, each formed of nano-crystals as described above, over oneanother in this order from the side closer to an excitation lightsource, it is possible to obtain white phosphorescence. By exciting theso produced white phosphor with the light emitted from a GaN-basedlight-emitting device, it is possible to realize an illuminationapparatus that emits white light.

[0026] As described above, extremely high quantum efficiency is obtainedwith some phosphors that are formed of fine crystals, i.e.,nano-crystals, of a III group nitride compound semiconductor which areso fine as to exhibit the quantum size effect. By exciting such aphosphor with a light-emitting device that emits light of wavelengths inthe range from 380 nm to 500 nm, it is possible to realize efficientdisplay apparatuses and illumination apparatuses.

[0027] The reason that the excitation wavelength is limited within therange from 380 nm to 500 nm is that, with a III group nitride compoundsemiconductor, it is difficult to produce a high-efficiency laser orlight-emitting diode (LED) that emits light of wavelengths of 380 nm orshorter, and that it is impossible to obtain blue phosphorescence withexcitation light of wavelengths of 500 nm or longer.

[0028] The studies described above have led to the present invention,according to which a light-emitting apparatus provided with a lightsource that emits primary light and a phosphor that absorbs at leastpart of the primary light emitted from the light source and emitssecondary light having a longer peak wavelength than the primary lightis characterized in that the phosphor is formed of fine-particlecrystals of a III-V group compound semiconductor, and that thefine-particle crystals each either have a volume of 2 800 nm³ or less ormeasure 14 nm or less in two directions perpendicular to the longestside thereof.

[0029] Preferably, the fine-particle crystals of the III-V groupcompound semiconductor (i.e., the nano-crystal phosphor) are given amultilayer structure as shown in FIG. 2 where a portion having a smallenergy band gap is surrounded with a portion having a great energy bandgap.

[0030] Typically, the light source that emits the primary light isrealized with one or more nitride-based III-V group compoundsemiconductor light-emitting devices, and these may be arranged in aone- or two-dimensional array.

[0031] Typically, these light-emitting devices are formed by growing anitride-based III-V group compound semiconductor on a substrate such asa nitride-based III-V group semiconductor substrate, sapphire substrate,SiC substrate, or ZnO substrate.

[0032] Preferably, the phosphor is formed of a III-V group compoundsemiconductor containing at least one III group element selected fromthe group consisting of Ga, Al, In, and B and at least one V groupelement including always N and in addition As or P as the case may be.It is particularly preferable that 50% or more of the III group elementsbe In and that 95% or more of the V group elements be nitrogen.

[0033] On the other hand, the primary light source uses a nitride-basedIII-V group compound semiconductor containing at least one III groupelement selected from the group consisting of Ga, Al, In, and B and atleast one V group element including always N and in addition As or P asthe case may be. Practical examples of such a nitride-based III-V groupcompound semiconductor include GaN, AlGaN, AlN, GaInN, AlGaInN, InN,GaNP, InNAs, InNP, and InGaNP.

[0034] In a light-emitting apparatus structured as described aboveaccording to the present invention, a phosphor is formed of crystalshaving a particle diameter equal to or smaller than twice the excitonBohr radius. Thus, by exciting this phosphor with light emitted from alight-emitting device using a nitride-based III-V group compoundsemiconductor, it is possible to enhance the quantum efficiency of thephosphor. Moreover, the small size of the crystals forming the phosphormakes it possible to obtain high resolution.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] This and other objects and features of the present invention willbecome clear from the following description, taken in conjunction withthe preferred embodiments with reference to the accompanying drawings inwhich:

[0036]FIG. 1 is a graph showing the dependence of the band gap energy ofZnS on the crystal size;

[0037]FIG. 2 is a sectional view showing a quantum dot of CdSe combinedwith an energy band diagram thereof;

[0038]FIGS. 3A and 3B are diagrams schematically showing an example ofthe structure of a light-emitting apparatus embodying the invention;

[0039]FIG. 4 is a diagram schematically showing another example of thestructure of a light-emitting apparatus embodying the invention;

[0040]FIG. 5 is a diagram schematically showing another example of thestructure of a light-emitting apparatus embodying the invention;

[0041]FIG. 6 is a diagram schematically showing another example of thestructure of a light-emitting apparatus embodying the invention;

[0042]FIG. 7 is a diagram schematically showing another example of thestructure of a light-emitting apparatus embodying the invention;

[0043]FIG. 8 is a sectional view schematically showing an example of thestructure of the laser used as the primary light source in alight-emitting apparatus embodying the invention;

[0044]FIG. 9 is a diagram showing the correlation between theoscillation wavelength of the laser used as the primary light source ina light-emitting apparatus embodying the invention and the lifetime ofthe light-emitting apparatus;

[0045]FIG. 10 is a sectional view schematically showing an example ofthe structure of the LED used as the primary light source in alight-emitting apparatus embodying the invention; and

[0046]FIG. 11 is a sectional view schematically showing an example of aphosphor having a multilayer structure for use in a light-emittingapparatus embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] Hereinafter, light-emitting apparatuses embodying the presentinvention will be described with reference to the drawings. In theembodiments described hereinafter, used as a primary light source is aIII-V group nitride semiconductor laser (hereinafter also referred tosimply as a “laser”) or a light-emitting diode (LED). In onelight-emitting apparatus embodying the invention, as shown in FIGS. 3Aand 3B, a laser 30 is used as a primary light source in combination witha phosphor 31. It is also possible to form a phosphorescent portion bydispersing fine-particle crystals of a III-V group compoundsemiconductor in resin. In that case, between the laser 30 and thephosphor 31, there is inserted a wavelength filter 32 that absorbs orreflects light of wavelengths shorter than 395 nm.

[0048] Moreover, in front of the phosphor 31 (on the exit side thereof),there is disposed a wavelength filter 33 that absorbs or reflects onlythe excitation laser light (the primary light emitted from the laser30). The laser 30 emits laser light of wavelengths in the range from 380nm to 500 nm. In a case where the phosphorescent portion is composed offine-particle crystals and resin, it is preferable that the laser havewavelengths in the range from 395 nm to 500 nm. The phosphor 31, whenexcited with the laser light emitted from the laser 30, emitsphosphorescence in the visible light region.

[0049] As shown in FIGS. 3A and 3B, the laser 30 and the phosphor 31 maybe arranged in close contact with each other, or may be arranged apartfrom each other. The phosphor 31 is arranged so as to face the laser 30so that the former can receive the laser light emitted from the latter.As shown in FIG. 3A, the laser light may be shone on glass or resin,such as acrylic resin, in which fine-particle crystals of a III-V groupcompound semiconductor are dispersed, or may be shone on a surface towhich a phosphor is applied or on which a coating thereof is formed. Thephosphor may even be given a shape like a sphere or optical fiber so asto achieve good optical coupling with the laser light. As shown in FIG.4, an optical system such as a lens 44 may be disposed between a primarylight source 40 and a phosphor 41.

[0050] As shown in FIG. 5, a primary light source may be coupled withthe core of an optical fiber 51 in which fine-particle crystals of aIII-V group compound semiconductor are dispersed. As shown in FIG. 6,the phosphorescence from a phosphor 61 may be condensed with a concavesurface mirror 65. As shown in FIG. 7, a light-emitting apparatus may beso structured that a laser used as a primary light source 70 is coupledwith a light guide plate 72 and a scattering plate 73 so that primarylight is extracted perpendicularly to the light guide plate 72, and thata phosphor 71 is disposed at where the extracted primary light isdirected so that secondary light is obtained as surface-emissionphosphorescence.

[0051] The laser may be of any type so long as it emits laser light ofwavelengths in the range from 380 to 500 nm. In a case where thephosphorescent portion is composed of fine-particle crystals and resin,it is preferable that the laser have wavelengths in the range from 395to 500 nm, because irradiating the resin with light of wavelengthsshorter than 395 nm causes deterioration of the resin. For this reason,it is preferable to dispose, between the laser and the phosphor, awavelength filter or the like that absorbs or reflects spontaneousemission light of wavelengths shorter than 395 nm included in the lightemitted from the laser.

[0052] Examples of the laser include a surface-emission-type device, astripe-shaped device, and a laser array having devices of either typearranged in an array.

[0053] Part of the laser light may be transmitted through the phosphorso as to be used along with the phosphorescence. However, in a casewhere a light-emitting apparatus embodying the invention is used forillumination purposes, letting laser light out to the outside world isundesirable because of its adverse effects on the human body. Thus, insuch a case, it is preferable to make the phosphor absorb all the laserlight, or to use a wavelength filter or the like to keep the laser lightfrom getting out to the outside world.

[0054] Preferable examples of the phosphor include one sintered, oneapplied to a plate-shaped member of a transparent material such asquartz, one dispersed in a glass-like substance such as glass or acrylicresin and then hardened, cut, and formed into a spherical, cylindrical,or fiber-like shape.

[0055] In a case where, as shown in FIG. 3A, a laser 30 and a phosphor31 are arranged apart from each other, the phosphor is a single memberor component. In this case, the phosphor 31 is, for example, atransparent member to which a phosphorescent substance is applied or onwhich a coating thereof is formed, or a glass member having aphosphorescent substance dispersed therein, and this phosphor 31 is usedin combination with a laser device.

[0056]FIG. 8 shows an example of the laser 30 used in the light-emittingapparatus shown in FIG. 3A. The GaN-based semiconductor laser 30 shownin this figure has a ridge structure, and has a cavity formed with anactive layer 84 cleaved at both ends so as to resonate and emit laserlight parallel to the active layer 84. Thus, the laser light oscillatesin the direction perpendicular to the plane of the figure, and thenexits from the laser 30 to strike the phosphor 31, causing it to emitphosphorescence to the outside world.

[0057] The laser 30 shown in FIG. 8 has, on top of a crystal substrate(for example, a GaN crystal substrate) 80, an n-GaN contact layer 81, ann-AlGaN clad layer 82, an n-GaN guide layer 83, an InGaN active layer84, a p-AlGaN evaporation prevention layer 85, a p-GaN guide layer 86, ap-AlGaN clad layer 87, and a p-GaN contact layer 88 laid on one another.On the top and bottom of these layers are formed a p-type electrode 89and an n-type electrode 800, respectively, and a SiO₂ layer 801insulates the p-type electrode 89 from the p-AlGaN clad layer 87 exceptin the ridge portion. As the primary light source, it is also possibleto use a surface-emission laser or an LED.

[0058] As the crystal substrate of the device used as the primary lightsource, it is possible to use any conventionally known substrate otherthan GaN so long as it permits growth of a GaN-based crystal thereon.Examples of such a crystal substrate include sapphire, SiC, Si, andquartz.

[0059] Light emission may be achieved with any structure so long as itis suitable for a laser to emit light and oscillate and for an LED toemit light. Examples of suitable structures include a doublehetero-junction structure, a structure including a SQW (single quantumwell), one including a MQW (multiple quantum well), and one including aquantum dot. With an LED, examples of suitable structures furtherinclude a simple pn-junction formed between two layers as a homo- orhetero-junction.

[0060] Hereinafter, practical examples of the present invention will bepresented.

EXAMPLE 1

[0061] A light-emitting apparatus structured as shown in FIG. 3A wasfabricated.

[0062] Primary Light Source

[0063] In this example, a stripe laser structured as shown in FIG. 8,i.e., one having a ridge structure, was fabricated as the primary lightsource. In the following description, the values given in cm⁻³ arecarrier densities.

[0064] Laser

[0065] As shown in FIG. 8, on top of a GaN substrate 80, an n-GaNcontact layer 81 (3 μm thick, 1×10¹⁸ cm⁻³), an n-Al_(0.1)Ga_(0.9)N cladlayer 82 (1 μm thick, 1×10¹⁸ cm⁻³), an n-GaN guide layer 83 (0.1 μmthick, 1×10¹⁸ cm⁻³), an In_(0.15)Ga_(0.85)N/In_(0.05)Ga_(0.95)N 3MQWactive layer 84, a p-Al_(0.15)Ga_(0.85)N evaporation prevention layer 85(0.02 μm thick, 1×10¹⁸ cm⁻³), a p-GaN guide layer 86 (0.1 μm thick,1×10¹⁸ cm⁻³), a p-Al_(0.1)Ga_(0.9)N clad layer 87 (0.6 μm thick, 1×10¹⁸cm⁻³), and a p-GaN contact layer 88 (0.1 μm thick, 1×10¹⁸ cm⁻³) werelaid on one another.

[0066] Next, by RIE, this multilayer structure was partially etched fromthe top surface thereof so that the p-AlGaN clad layer 87 was exposedexcept for a portion thereof. Then, on top, a SiO₂ (0.3 μm) layer 801was laid, and, further on top, a p-type electrode 89 (Pd/Mo/Au) wasformed. Moreover, on the bottom surface of the multilayer structure, ann-type electrode 800 (Ti/Al) was formed. The oscillation wavelength ofthe apparatus was 405 nm.

[0067] Phosphor

[0068] InN nano-crystals having volumes of 8 nm³ to 1 000 nm³ (2 nm to10 nm along each side) were synthesized through chemical synthesis, andthen, by a sol-gel process, acrylic resin having the InN nano-crystalsdispersed therein was formed into a film (3 μm thick). The reason thatthe volume varied from 8 nm³ to 1 000 nm³ (2 nm to 10 nm along eachside) was that the volume that produced the same phosphorescencewavelength varied according to the conditions of synthesis.

[0069] Evaluation

[0070] When the laser of the above light-emitting apparatus wasenergized to cause laser oscillation, phosphorescence of a wavelength635 nm was obtained with energy conversion efficiency of 80 [lm/W].Assuming that the length of time over which the energy conversionefficiency fell half its initial value was the lifetime, the lifetimewas about 10 000 hours.

[0071] For comparison, the above light-emitting apparatus was operatedwith the wavelength filter 32 shown in FIG. 3A removed. In this case,the lifetime, over which the energy conversion efficiency fell half itsinitial value, was 1 000 hours.

EXAMPLE 2

[0072] A light-emitting apparatus structured as shown in FIG. 3B wasfabricated.

[0073] Primary Light Source

[0074] As the primary light source, the same laser as used in Example 1was used.

[0075] Phosphor

[0076] InN nano-crystals having volumes of 3.375 nm³ to 64 nm³ (1.5 nmto 4 nm along each side) were grown by laser ablation, and then, on topof the InN, GaN was grown by laser ablation in a similar manner toproduce InN/GaN nano-crystals having a quantum well structure as shownin FIG. 2. Then, acrylic resin having these nano-crystals dispersedtherein was formed into a cylindrical shape. The reason that the volumevaried from 3.375 nm³ to 64 nm³ (1.5 nm to 4 nm along each side) wasthat the volume that produced the same phosphorescence wavelength variedaccording to the conditions of growth.

[0077] Evaluation

[0078] When the laser of the above light-emitting apparatus wasenergized to cause laser oscillation, phosphorescence of a wavelength520 nm was obtained with energy conversion efficiency of 120 [lm/W].

[0079] For comparison, a laser that oscillated at a different wavelengthwas produced and used as the excitation light source, and thecorrelation between the wavelength of the excitation light source andthe lifetime, over which the energy conversion efficiency fell to halfits initial value, was evaluated. Used as the wavelength filter 32 wasone that absorbed or reflected spontaneous emission light of wavelengthsshorter than the oscillation wavelength. FIG. 9 shows the relationshipbetween the wavelength and the life time, over which the energyconversion efficiency fell to half its initial value. With glass resin,the lifetime decreased as the wavelength shortened below 380 nm; withacrylic resin, the lifetime decreased as the wavelength shortened below395 nm. This decrease in the lifetime was due to, with glass resin, thedeterioration of the laser used as the excitation light source and, withacrylic resin, the deterioration of the acrylic resin.

EXAMPLE 3

[0080] A light-emitting apparatus structured as shown in FIG. 4 wasfabricated.

[0081] Primary Light Source

[0082] In this example, an LED structured as shown in FIG. 10 wasfabricated. In the following description, the values given in cm⁻³ arecarrier densities.

[0083] Laser

[0084] As shown in FIG. 10, on top of a sapphire substrate 90, a bufferlayer (not shown) was grown, and then an n-GaN contact layer 91 (3 μmthick, 1×10¹⁸ cm⁻³), an In_(0.12)Ga_(0.87)N/GaN 5MQW active layer 92, ap-Al_(0.15)Ga_(0.85)N evaporation prevention layer 93 (0.02 μm thick,1×10¹⁸ cm⁻³), a p-GaN contact layer 94 (0.2 μm thick, 1×10¹⁸ cm⁻³) werelaid on one another. Next, by RIE, this multilayer structure waspartially etched from the top surface thereof so that the n-GaN contactlayer 91 was exposed except for a portion thereof. Then, on top, ann-type electrode 95 (Ti/Al) was formed. Moreover, on top of the p-GaNcontact layer 94, a p-type transparent electrode 96 (Pd; 0.008 nm) wasformed, and on top of a portion thereof, a p-type electrode 97(Pd/Mo/Au) was formed.

[0085] Phosphor

[0086] Three types of InN nano-crystals were produced by laser ablation,namely those having volumes of 8 nm³ to 27 nm³ (2 nm to 3 nm along eachside), those having volumes of 10.7 nm³ to 64 nm³ (2.2 nm to 4 nm alongeach side), and those having volumes of 17.6 nm³ to 512 nm³ (2.6 nm to 8nm along each side). Then, as shown in FIG. 11, these three types ofnano-crystals were dispersed in acrylic resin in three layers in such away that the size of the InN nano-crystals decreases from the entranceside to the exit side of the primary light. The thicknesses of thesethree layers were, with consideration given to how each layer absorbedthe primary light, so adjusted that the phosphorescence emitted from theindividual layers and mixed together was white.

[0087] Evaluation

[0088] When the LED of the above light-emitting apparatus was energizedto emit light, white light was obtained with energy conversionefficiency of 60 [lm/W].

EXAMPLE 4

[0089] A light-emitting apparatus structured as shown in FIG. 5 wasfabricated.

[0090] Primary Light Source

[0091] As the primary light source, a stripe laser of an embedded typewas used.

[0092] Phosphor

[0093] In_(0.95)Ga_(0.05)N nano-crystals having volumes of 125 nm³ to343 nm³ (5 nm to 7 nm along each side) were synthesized through chemicalsynthesis, and on the InGaN, AlN was synthesized through chemicalsynthesis in a similar manner to produce In_(0.95)Ga_(0.05)N/AlNnano-crystals having a quantum well structure as shown in FIG. 2. Then,an optical fiber was produced of which the core had these nano-crystalsdispersed therein.

[0094] Evaluation

[0095] When the laser of the above light-emitting apparatus wasenergized to cause laser oscillation, phosphorescence of a wavelength600 nm was obtained with energy conversion efficiency of 120 [lm/W].

EXAMPLE 5

[0096] A light-emitting apparatus structured as shown in FIG. 6 wasfabricated.

[0097] Primary Light Source

[0098] As the primary light source, a surface-emission laser was used.

[0099] Phosphor

[0100] In_(0.98)Ga_(0.02)N_(0.99)P_(0.01) nano-crystals having volumesof 343 nm³ to 512 nm³ (7 nm to 8 nm along each side) were synthesizedthrough chemical synthesis, and then, by a sol-gel process, glass havingthe InGaNP nano-crystals dispersed therein was formed into a film (1 mmthick).

[0101] Evaluation

[0102] When the laser of the above light-emitting apparatus wasenergized to cause laser oscillation, phosphorescence of a wavelength560 nm was obtained with energy conversion efficiency of 100 [lm/W].

EXAMPLE 6

[0103] A light-emitting apparatus structured as shown in FIG. 7 wasfabricated.

[0104] Primary Light Source

[0105] As the primary light source, the same laser as used in Example 1was used.

[0106] Phosphor

[0107] InN nano-crystals having volumes of 343 nm³ to 1000 nm³ (7 nm to10 nm along each side) were synthesized through chemical synthesis, andthen, by a sol-gel process, a glass film (10 μm thick) having the InNnano-crystals dispersed therein was formed on a light guide plate.

[0108] Evaluation

[0109] When the laser of the above light-emitting apparatus wasenergized to cause laser oscillation, phosphorescence of a wavelength550 nm was obtained with energy conversion efficiency of 100 [lm/W].

[0110] Obviously, many modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced other than as specifically described.

What is claimed is:
 1. A light-emitting apparatus comprising: a lightsource that emits primary light; and a phosphor that absorbs at leastpart of the primary light emitted from the light source and emitssecondary light having a longer peak wavelength than the primary light,wherein the phosphor is formed of fine-particle crystals of a III-Vgroup compound semiconductor, the fine-particle crystals each having avolume of 2 800 nm³ or less.
 2. A light-emitting apparatus as claimed inclaim 1, wherein, of III group elements and V group elements containedin the III-V group compound semiconductor, 50% or more of the III groupelements is indium, and 95% or more of the V group elements is nitrogen.3. A light-emitting apparatus as claimed in claim 1, wherein thefine-particle crystals of the III-V group compound semiconductor have apredetermined volume distribution so that the secondary light emittedfrom the phosphor has a wavelength distribution corresponding to thevolume distribution of the fine-particle crystals.
 4. A light-emittingapparatus as claimed in claim 1, wherein the III-V group compoundsemiconductor is a nitride semiconductor, and the fine-particle crystalsthereof are each composed of a single portion having a uniform energyband gap.
 5. A light-emitting apparatus as claimed in claim 1, whereinthe III-V group compound semiconductor is a nitride semiconductor, andthe fine-particle crystals thereof are each composed of a first portionand a second portion that encloses the first portion and that has agreater energy band gap than the first portion.
 6. A light-emittingapparatus as claimed in claim 1, wherein the fine-particle crystals ofthe III-V group compound semiconductor are dispersed in glass, and thepeak wavelength of the primary light emitted from the light source is ina range from 380 nm to 500 nm, both ends inclusive.
 7. A light-emittingapparatus as claimed in claim 1, wherein the fine-particle crystals ofthe III-V group compound semiconductor are dispersed in resin, and thepeak wavelength of the primary light emitted from the light source is ina range from 395 nm to 500 nm, both ends inclusive.
 8. A light-emittingapparatus as claimed in claim 1, wherein, in an optical path from thelight source to the phosphor, a filter is provided that cuts off lightof wavelengths shorter than 395 nm.
 9. A light-emitting apparatus asclaimed in claim 1, wherein, in an optical path of the secondary lightemitted from the phosphor, a filter is provided that cuts off theprimary light emitted from the light source.
 10. A light-emittingapparatus as claimed in claim 1, wherein the light source includes alight-emitting device using a nitride-based III-V group compoundsemiconductor.
 11. A light-emitting apparatus comprising: a light sourcethat emits primary light; and a phosphor that absorbs at least part ofthe primary light emitted from the light source and emits secondarylight having a longer peak wavelength than the primary light, whereinthe phosphor is formed of fine-particle crystals of a III-V groupcompound semiconductor, the fine-particle crystals each measuring 14 nmor less in two directions perpendicular to a longest side thereof.
 12. Alight-emitting apparatus as claimed in claim 11, wherein, of III groupelements and V group elements contained in the III-V group compoundsemiconductor, 50% or more of the III group elements is indium, and 95%or more of the V group elements is nitrogen.
 13. A light-emittingapparatus as claimed in claim 11, wherein the fine-particle crystals ofthe III-V group compound semiconductor have a predetermined volumedistribution so that the secondary light emitted from the phosphor has awavelength distribution corresponding to the volume distribution of thefine-particle crystals.
 14. A light-emitting apparatus as claimed inclaim 11, wherein the III-V group compound semiconductor is a nitridesemiconductor, and the fine-particle crystals thereof are each composedof a single portion having a uniform energy band gap.
 15. Alight-emitting apparatus as claimed in claim 11, wherein the III-V groupcompound semiconductor is a nitride semiconductor, and the fine-particlecrystals thereof are each composed of a first portion and a secondportion that encloses the first portion and that has a greater energyband gap than the first portion.
 16. A light-emitting apparatus asclaimed in claim 11, wherein the fine-particle crystals of the III-Vgroup compound semiconductor are dispersed in glass, and the peakwavelength of the primary light emitted from the light source is in arange from 380 nm to 500 nm, both ends inclusive.
 17. A light-emittingapparatus as claimed in claim 11, wherein the fine-particle crystals ofthe III-V group compound semiconductor are dispersed in resin, and thepeak wavelength of the primary light emitted from the light source is ina range from 395 nm to 500 nm, both ends inclusive.
 18. A light-emittingapparatus as claimed in claim 11, wherein, in an optical path from thelight source to the phosphor, a filter is provided that cuts off lightof wavelengths shorter than 395 nm.
 19. A light-emitting apparatus asclaimed in claim 11, wherein, in an optical path of the secondary lightemitted from the phosphor, a filter is provided that cuts off theprimary light emitted from the light source.
 20. A light-emittingapparatus as claimed in claim 11, wherein the light source includes alight-emitting device using a nitride-based III-V group compoundsemiconductor.
 21. A phosphor comprising: fine-particle crystals of aIII-V group compound semiconductor, wherein the fine-particle crystalseach have a volume of 2 800 nm³ or less.
 22. A method of producing aphosphor, comprising the step of: producing, from materials containing aIII group element and a V group element and through chemical synthesis,a phosphor formed of a III-V group compound semiconductor in a form offine-particle crystals each having a volume of 2 800 nm³ or less.
 23. Amethod of producing a phosphor, comprising the step of: producing, byusing a III-V group compound semiconductor as a material and by laserablation, a phosphor formed of fine-particle crystals each having avolume of 2 800 nm³ or less.
 24. A phosphor comprising: fine-particlecrystals of a III-V group compound semiconductor, wherein thefine-particle crystals each measure 14 nm or less in two directionsperpendicular to a longest side thereof.
 25. A method of producing aphosphor, comprising the step of: producing, from materials containing aIII group element and a V group element and through chemical synthesis,a phosphor formed of a III-V group compound semiconductor in a form offine-particle crystals each measuring 14 nm or less in two directionsperpendicular to a longest side thereof.
 26. A method of producing aphosphor, comprising the step of: producing, by using a III-V groupcompound semiconductor as a material and by laser ablation, a phosphorformed of fine-particle crystals each measuring 14 nm or less in twodirections perpendicular to a longest side thereof.